The use of pleated paper elements in both oil filters and in air filters on motor vehicle internal combustion engines to remove particulate materials is well-known, and the manufacture of filter paper of a porosity suitable for such elements has reached such a degree of excellence that filter paper is readily available commercially from which pleated paper elements can be produced having filtering efficiencies of at least 97% when tested using a standard fine test dust.
It is desirable, where possible, to produce pleated paper filter elements having as high a dust-holding capacity as possible without any appreciable loss in filtering efficiency occurring, since a high dust-holding capacity in a pleated paper filter element can materially improve the service life of that filter. This is particularly the situation with pleated paper filter elements for use in air filters.
The commercial manufacture of filter paper involves the production of a paper pulp from a source of cellulose and the subsequent deposition and dewatering of that pulp upon wire screens in order to obtain a coherent sheet of filter paper. In such a sheet of filter paper, the side of the paper which, during manufacture, was adjacent to the wire screens of the paper-making machine, and known as the "wire-side" of the paper, is more compacted than is the opposite side of the paper, and customarily is distinctively marked as such by the paper manufacturer, since the manufacture of pleated paper filter elements from such paper involves arranging for the wire-side of the paper to form the clean side of the filter element. The clean side of a filter element is the side at which the filtered fluid leaves the filter element. By careful control of the constituents of the paper pulp and the processing steps used, it is possible to obtain virtually any desired degree of overall porosity in the manufactured filter paper, but it is generally found that the higher the degree of porosity produced in the filter paper, the lower is the overall mechanical strength of the filter paper. Consequently, there is a limit on the degree of porosity available in filter paper produced from cellulose fibers and suitable for the manufacture of pleated paper filter elements.
Much work has recently been carried out, particularly by Japanese paper manufacturers, on the production of filter papers in order to produce a layered structure in the paper so that the structure contains one or more layers of different fibrous material in or adjacent the opposite side of the paper to the wire-side. Such layered structures are alleged to improve the dust-holding capacity of pleated paper filter elements manufactured from such filter papers. The production of such layered structures, however, necessarily complicates the production procedure, and involves the use of more expensive ingredients than those normally used in filter paper manufacture, so making the finished product substantially more expensive to use in the manufacture of pleated paper filter elements.
An alternative approach to a layered structure in the paper, used in the production of a disposable dust container from single ply filter paper for use in vacuum cleaners, is disclosed in GB-A-1,197,250. GB-A-1,197,250 discloses the concept of roughening one side of the filter paper in order to produce a layered structure in the paper comprising an inner thicket of fibers superimposed on the remaining thickness of the paper, the original unroughened paper having an initial thickness of around 0.2 mm., and the remaining thickness of the paper after roughening being less than 0.2 mm. The roughening procedure comprises the mechanical abrasion of one side of the single ply paper with, for example, rotating brushes, to wear away portions of the surface of said one side in order to create said inner thicket of fibers, and produces a disposable dust container formed from a single ply paper which displays an improved porosity compared to a similar dust container made from single ply unroughened filter paper. It appears clear from the disclosure made in GB-A-1,197,250 that the extent of the roughening procedure must be carefully controlled to ensure that the part of the paper which is not roughened is sufficiently strong for the bag to serve as a place for storage for collected dust during vacuum cleaning, and also when the bag is removed from the vacuum cleaner for disposal.